In recent years, advances in technology, as well as ever evolving tastes in style, have led to substantial changes in the design of automobiles. One of the changes involves the power usage and complexity of the various electrical systems within automobiles, particularly alternative fuel vehicles, such as hybrid, electric, and fuel cell vehicles. Many of these vehicles use electric motors to provide traction power to the vehicle.
For induction motors, the speed of the rotor and the speed of the rotating magnetic field in the stator must be different, a concept known as slip, in order to induce current. In order to operate the induction motor at its highest efficiency, the slip is controlled using feedback control loops. In conventional control systems, as the rotor speed increases, the rotor approaches a base speed (or rated speed), where the voltage across the motor terminals reaches a value at which no more current can be provided to the motor. In order to operate the motor at higher speeds than the base speed, a technique known as flux weakening, controlled by non-torque generating current is employed.
Accordingly, field-oriented control methods have been developed to control the torque generating current supplied to the induction motor separately from the non-torque generating current. These methods use the relative position and speed of the rotor to maintain a desired relationship between the stator flux and rotor flux. The non-torque generating current is adjusted based on the speed of the rotor and the flux characteristics of the induction motor. By compensating for the undesired flux, field-oriented control can be used to improve efficiency, the motor transient response, and tracking of the torque command at speeds higher than the base speed. As a result of the improved performance, induction motors and drive systems may be appropriately sized for an application, thereby lowering cost and improving overall efficiency.
Most field-oriented control methods for induction motors utilize incremental encoders to measure the relative position and speed of the rotor. Typically, these encoders are either magnetic or optical. For automotive environments, packaging space is often at a premium and the encoders are often exposed to demanding environmental conditions. For example, the operating temperature may range from −40° C. to 150° C., which exceeds the operating temperature ratings for most optical encoders. While magnetic encoders may be able to tolerate automotive temperatures, they often cannot sustain operation when exposed to vibration forces and frequencies encountered in automotive applications. Furthermore, in order to achieve high-levels of accuracy, magnetic encoders must be implemented in a large physical size, which is undesirable from a packaging and automotive design perspective.